application of microphonics measurement system in SHINE horizontal testing

Background In the Shanghai HIgh repetitioN rate XFEL and Extreme light facility (SHINE), superconducting cavities at frequencies of 1.3 GHz and 3.9 GHz are primarily used to accelerate the electron beam. Considering both the power coupling of the RF source and the performance of the high-power coupler during the overall operation of SHINE, the loaded quality factor (Q_L) of the superconducting cavities is set to be greater than 4.12×10⁷. This results in an extremely narrow operating bandwidth for the superconducting cavity system, making it susceptible to external disturbances. Consequently, the accelerating cavity voltage exhibits fluctuations, which severely impact the beam energy stability and degrade the quality of the beam.

Purpose The main task of this study is to develop a measurement system capable of, during superconducting cavity operation, determining both the vibration frequency of external disturbance sources and the magnitude of the resulting frequency variation of the superconducting cavity.

Methods The measurement system is built around NI PXIe instruments and programmed using LabVIEW for discrete data acquisition. The SEL (Symmetric Extension of Local) morphological discrete frequency variation principle is employed for data processing, followed by a Fourier transform to display the oscillation frequency and frequency deviation of the vibration sources.

Results The system was verified by measuring the 1.3 GHz and 3.9 GHz cavities. The measurement error of frequency deviation was evaluated as a function of different oscillation frequencies and frequency deviations. The results show that the frequency deviation error increases with larger frequency deviation values and is proportional to the oscillation frequency. The precision of frequency deviation under different signal powers was also measured, yielding a frequency deviation error of 0.002 Hz. Using the system, the frequency deviations of eight 1.3 GHz cavities in the CM02 module and eight 3.9 GHz cavities in the HCM01 module were measured on a horizontal test platform. Additionally, the system was used to evaluate the precision of piezo actuators.

Conclusions This system can detect the oscillation frequency and frequency deviation caused by external disturbance sources on the superconducting cavity modules, meeting the design goals of the system. It serves as a prerequisite exploration for designing subsequent microphonics suppression schemes.